X-ray imaging technology provides a non-invasive technique for visualizing the internal structure of an object of interest by exposing the object to high energy electromagnetic radiation (i.e., X-rays). X-rays emitted from a radiation source pass through the object and are absorbed at varying levels by the internal structures of the object. As a result, X-ray radiation exiting the object is attenuated according to the various absorption characteristics of the materials which the X-rays encounter.
The absorption characteristics of the object of interest may be captured by placing the object between a high energy electromagnetic radiation source and an image recording medium. As radiation from the source passes through the object, the radiation impinges on the image recording medium with an intensity related to the attenuation caused by the different absorption characteristics of the object. The impinging radiation causes a change in the image recording medium that is proportional to the radiation intensity, thereby storing information about the internal structure of the object. The image recording medium may then be processed to recover the stored information by, for instance, converting it into digital form. Common types of image recording media include sheet film, phosphor media, etc.
Phosphor plate or screen technology has emerged as a valuable image recording media for computed radiography (CR). When electromagnetic radiation, such as X-ray radiation, impinges on a phosphor plate, the radiation interacts with the phosphor lattice of the plate. The phosphor molecules in the plate store energy proportional to the intensity of the impinging radiation. This energy can later be released by scanning the plate with a laser to excite the phosphors in the plate (i.e., by causing the phosphor molecules to fluoresce). The excited phosphor molecules release radiation that can be detected, quantified and stored as values representing pixels in an image. Apparatus adapted to releasing and detecting information from phosphor plates are often referred to as computed radiography (CR) scanners, CR image readers, or more generally referred to as CR systems.
The radiation provided to obtain information from an image recording medium is referred to as “stimulating radiation” and describes any electromagnetic radiation capable of exciting an image recording medium. In the context of CR systems, stimulating radiation is commonly provided as a laser having power and frequency characteristics capable of exciting phosphor molecules in a phosphor plate. The stimulating radiation is typically provided to the image recording medium in a generally planned path. This process is referred to herein as “scanning.” One method of scanning includes logically dividing an area of an image recording medium into a plurality of pixel regions. Each pixel region may correspond to a pixel in a resulting image acquired from the medium. For example, FIGS. 1A and 1B illustrate a portion of an image recording medium 100 divided into a regular Cartesian grid 10 (denoted by the dotted lines) labeled as A–F on the y-axis and 1–6 on the x-axis, the grid forming a plurality of rectangular pixel regions.
An image typically represents intensity as a function of space. The term “intensity” refers generally to a magnitude, degree and/or value at some location in the image. For example, in an X-ray image, the pixel intensity generally represents the absorption characteristics of scanned material at a particular location in space and may be related, among other characteristics, to the density of the material. An image may be formed by assigning an intensity value to each of the pixel regions logically assigned to an image recording medium. For example, where the image recording medium (e.g., medium 100) is a phosphor plate, an intensity value may be assigned to each pixel region based on the amount of photonic energy assumed to have been released from phosphor molecules located within the respective pixel region in response to stimulating radiation applied during scanning.
Two examples of a scanning path along grid 10 include raster XY and raster YX order. In raster YX order illustrated in FIG. 1A, the scanner traverses the grid across the domain of the y-axis in a direction from A to F while holding the x-coordinate constant, thus scanning along a vertical scan trace, for example, scan trace 15a. The x-coordinate may then be incremented and the y domain scanned again along the next scan trace 15b. This process may be repeated until a desired area of the image recording medium has been traversed. Similarly, a raster XY scan (illustrated in FIG. 1B) traverses the grid across the x-domain and then increments the y-coordinate as indicated by horizontal scan traces 15a′–15f′. 
The term scan trace (or simply trace) refers to a path over a surface or portion of an image recording medium. A scan trace may be a path that varies over a range of a first dimension of an image recording medium while remaining substantially constant over another dimension, or may vary simultaneously over more than one dimension of the image recording medium. Image information obtained from applying radiation along a scan trace may correspond to a plurality of pixel intensities over one dimension of the resulting image. For example, image information obtained along a scan trace may correspond to a single complete row or column of pixels in an image. However, image information obtained along some scan traces (e.g., a helical scan trace) may correspond to more than one row or column of pixels. A scan trace may be a line (e.g., scan traces 15a–15b in FIG. 1), an arc (e.g., as described in detail in connection with FIG. 7), a helix or any other suitable path over the surface of an image recording medium.
The resolution of an image obtained from an image recording medium may depend in part on a scanners ability to excite intended regions of an image recording medium in isolation, and then to resolve the location from where detected energy was released. In general, the smaller the region that can be stimulated in isolation and detected, the greater the resolution of the resulting image. Accordingly, the dimensions of a pixel region (e.g., a pixel width and a pixel length) of the logical grid applied to the image recording medium is inversely related to the resolution.
CR image readers often employ a laser beam to excite regions of a phosphor plate that have been exposed to X-ray radiation. Conventional image readers typically provide the laser over a range of a first dimension of the phosphor plate in a continuous fashion. For example, a laser beam may be applied continuously along the y-axis in a raster YX scan along an essentially linear scan trace. The photons released by the phosphors in the plate may then be detected, for example, by a photomultiplier tube and converted into a portion of a digital image. The laser may then be translated by one pixel width over a second dimension (e.g., the x-axis in a raster YX scan) and traversed along a subsequent scan trace. Since the phosphor plate is stimulated along a trace in a continuous fashion, the photons emitted from the phosphor plate along a scan line will be released in a substantially continuous fashion. A detector adapted to sense the emitted photons, therefore, will respond with a generally continuous output, referred to as a detection signal. That is, detection signals generated by a detector in conventional CR systems will be continuous in nature. The generally continuous detection signals must then be digitized to form the individual pixels in the image. This may be achieved in a number of ways including integrating the continuous signal over predetermined intervals, sampling the continuous signals at a predetermined sampling interval, etc. The value obtained by processing the detection signals may then be converted to a digital number representing the corresponding density as a brightness value in some desired range suitable for display.